[425HP409]

Flint, a company in Arkansas makes an adapter that will connect to your electronic transmission and convert the signal through a box and drive a cable that can be hooked up to your original speedometer. It s called a Cable X. The company's website is www.abbott-tach.com
I have used their products for years and it is really good. I installed 285-75r16 tires on my 01 GMC truck from the original 245-75R16s and installed their ERA (electronic ratio adapter) to get my speedo to read right on the money. I have also used their Cable X on a customers vehicle after installing a 4l60E trans.